Conventionally, there is known to be a method of providing a display device with multiple photosensors in the display panel and providing an input function using a touch panel, pen input, a scanner, or the like. The influence of the light environment needs to be eliminated in order to apply this method to mobile devices that are used in various light environments. In view of this, there is also known to be a method of removing a component that is dependent on the light environment from a signal detected by a photosensor in order to obtain the signal that was originally intended to be input.
Japanese Patent No. 4072732 discloses an input/output device that has photoreception elements provided in correspondence with individual display elements, in which the backlight is lit and extinguished one time in one frame period, and resetting and readout are performed on the photoreception elements in a line-sequential manner so as to acquire the light amount in the backlight lit period and the light amount in the backlight extinguished period in one frame period from all of the photoreception elements.
FIG. 25 is a diagram showing backlight lighting and extinguishing timings and photoreception element resetting and readout timings that are disclosed in Japanese Patent No. 4072732.As shown in FIG. 25, the backlight is lit in the first half of a frame period and extinguished in the last half. In the backlight lit period, resetting is performed on the photoreception elements in a line-sequential manner (solid arrow), and then readout from the photoreception elements is performed in a line-sequential manner (dashed arrow). In the backlight extinguished period as well, resetting and readout are performed on the photoreception elements in a similar manner.
Japanese Patent No. 3521187 discloses a solid-state imaging device that includes the unit photoreception portion shown in FIG. 26. The unit photoreception portion shown in FIG. 26 includes one photoelectric conversion portion PD and two charge accumulation portions C1 and C2. When both external light and light from a light-emitting means that has been reflected by an object are received, a first sample gate SG1 switches on, and charge generated by the photoelectric conversion portion PD is accumulated in the first charge accumulation portion C1. When only external light is received, a second sample gate SG2 switches on, and charge generated by the photoelectric conversion portion PD is accumulated in the second charge accumulation portion C2. The amount of light from the light-emitting means that was reflected by the object can then be obtained by obtaining the difference between the amounts of charge accumulated in the two charge accumulation portions C1 and C2.
With a display device that has multiple photosensors provided in a display panel, readout from the photosensors is generally performed in a line-sequential manner. Also, backlights for mobile devices are lit at the same time and extinguished at the time over the whole screen.
With the input/output device disclosed in Japanese Patent No. 4072732, the backlight is lit and extinguished one time in one frame period, resetting and readout are performed in non-overlapping periods in the backlight lit period, and resetting and readout are performed in non-overlapping periods in the backlight extinguished period as well. For this reason, readout from the photoreception elements needs to be performed in ¼ of the frame period (e.g., in 1/240 sec if the frame rate is 60 frames/sec). However, performing readout at such a high speed is actually quite difficult.
Also, there is a deviation of ½ of a frame period between the period in which the photoreception elements detect light in the backlight lit period (B1 shown in FIG. 25), and the period in which the photoreception elements detect light in the backlight extinguished period (B2 shown in FIG. 25). The ability to follow motion input therefore fluctuates according to the input direction. Also, with this input/output device, readout starts immediately after the completion of resetting, and resetting starts immediately after the completion of readout. The durations of and intervals between the backlight lit period and the backlight extinguished period therefore cannot be determined freely.
Also, with this input/output device, the amount of light in the backlight lit period and the amount of light in the backlight extinguished period are detected by the same photoreception element. For this reason, when the amount of light in the backlight lit period is detected in a certain photoreception element, the detection of the amount of light in the backlight extinguished period cannot be started in that photoreception element until the previously detected amount of light is read out from the photoreception element.